1. Field of the Invention
This invention relates generally to a signal converting circuit and, more particularly, is directed to a signal converting circuit for converting a pair of differential input signals to a single-ended output signal.
2. Description of the Prior Art
Signal converting circuits, for example, of the differential amplifier type, for converting a pair of differential input signals to a single-ended output signal are well-known in the art. One such signal converting circuit includes a pair of transistors which form a differential amplifier. A bias current source is supplied to the commonly connected emitters of the pair of transistors and differential input signals are supplied to the respective inputs of the transistors. Accordingly, differential output currents are produced by the transistors at the collectors thereof and are used to generate a single-ended output signal.
With such circuit, however, in order to avoid any deleterious effect on the differential output currents, it is undesirable to directly utilize such differential output currents in the next stage. In this regard, a current mirror circuit is conventionally used with such differential amplifier circuit whereby changes in at least one of the differential output currents is reflected by identical changes in the output current of the current mirror circuit. For example, one such current mirror circuit includes a diode connected between a voltage source and the output of one of the transistors of the differential amplifier and a PNP transistor having its base-emitter path connected in parallel with the diode. The collector of the PNP transistor serves as the output terminal for producing an output current corresponding to that produced at the output of the one transistor of the differential amplifier.
However, such circuit has proven to be disadvantageous if the emitter area of the PNP transistor of the current mirror circuit is N times larger than the area of the cathode of the diode used therein. In such case, the current produced at the collector of the transistor of the current mirror circuit is equal to N times the current flowing through the diode. This, of course, is undesirable and results in an increased current being supplied to the next stage of circuitry and an increased power consumption from the voltage source connected to the diode of the current mirror circuit. In this manner, it becomes exceedingly difficult to amplify the input signals. Further, in order to vary the gain or amplification of the signal converting circuit, it is necessary to increase the value of the constant current from the current source of the differential amplifier, which is also undesirable.